1. Field of the Invention
The invention relates to a slot die for producing webs of a foamed plastics material, into which die plastics material which has been plasticized under elevated pressure in an extruder and charged with a blowing agent enters, and through the cooled die gap of which the plastics material is extruded by foaming.
2. Discussion of the Prior Art
DE 17 79 374 has already disclosed, for the extrusion of articles of foamed thermoplastic material, having the slot die open directly into a calibrating device, which has pressure locks in which calibrating rollers are arranged. This is intended to achieve the effect that the extruded articles can be calibrated very exactly, for which purpose it is necessary to control the pressure in the pressure chambers correspondingly.
U.S. Patent No. 2,857,625 has also already disclosed arranging a cooled pair of rollers behind the die gap of a die. Here, however, free foaming of the plastics material already takes place before entry into the roller nip, since atmospheric pressure prevails in the space between. the die gap and the pair of rollers.
The known solutions have the disadvantage that, after leaving the die gap of the slot die, the plastics material foams before compressed outer layers are able to build up on both sides of the web, with the result that blowing agent can escape prematurely. This is disadvantageous for intensive foaming of the web. Added to this is the fact that the transverse expansion causes waves to form in the web between when it leaves the slot die and when it reaches the roller nip. In particular when alternative low-boiling blowing agents or blowing agents such as for example N2 and O2 are used, it is also necessary to work with higher blowing-agent pressures in the slot die. To ensure acceptable foaming behavior in this case, it is also necessary to cool the plastics material at the die lips of the slot die so intensively that outer layers are formed, in order in this way to prevent uncontrolled foaming of the emerging material. However, limits are prescribed here for the cooling, since the necessary intensive cooling may lead to the plastics material stagnating in the die lips. This causes an undefined constriction of the die gap, whereby the pressure and the flow in the die gap are adversely influenced.
The invention is based on the object of avoiding the disadvantages mentioned and of providing a slot die with which it is possible to produce webs of foamed plastics material accompanied by continuous foaming of the material.
This object is achieved according to the invention by the features of claim 1. Further development of the invention can be taken from the subclaims. Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in the die zap being formed by a pair of rollers mounted in a laterally sealed manner in the slot die.
The fact that the die gap is formed by a pair of rollers mounted in a laterally sealed manner in the slot die achieves the effect that compressed outer layers of relatively high-viscosity plastics material already form on the emerging plastics material when it comes into contact with the surfaces of the rollers, as a result of cooling down, ahead of the die gap, i.e. before the start of the foaming operation. Since the pairs of rollers are rotating, the effect is obtained that the outer layers that are in contact are moved by the rollers of the pairs of rollers through the die gap, without stagnation and/or freezing occurring. Moreover, the plastics melt flowing onto the rollers always comes into contact with new regions of the surfaces of the rollers, which positively influences the temperature control of the melt. There is consequently the possibility of intensively cooling the emerging material, so that a formation of outer layers already occurs ahead of the die gap before the start of foaming. When it subsequently emerges from the die gap, foaming of the plastics material then takes place, but the outer layers keep the blowing agent in the foam core, so that intensive internal foaming of the web occurs. Consequently, highly foamed materials can be produced. In addition, the outer layers prevent waves from forming in the web, since transverse expansion cannot occur.
According to a preferred embodiment of the invention, the pair of rollers is provided on each of the two sides with a sealing thread, acting as a melt seal, as a lateral sealing means. The sealing thread may act either as a return conveying thread for the extruded melt or else as a sealing thread of an active melt seal, a sealant then being fed in from the outside in the sealing thread. The sealing thread ensures that no pressure drop takes place ahead of the die gap while the slot die is in operation.
It may also be provided within the scope of the invention that the rollers of the pair of rollers are of a frustoconical design and are displaceable axially with respect to each other. Such a design of the rollers makes it possible to set the thickness of the die gap by mutual axial displacement. The axial adjustment of the rollers is made possible in particular also by the sealing thread provided according to the invention.